The present invention relates to a composition and method for controlling exhaust emissions, especially nitrogen oxides emissions, from combustion engines. More specifically, the present invention relates to a series of composite metal oxides that significantly improve the sulfur tolerance of a Lean NOx Trap (LNT).
An approach for treating the NOx emissions from lean-burn engines is the use of NOx traps. A LNT typically contains precious metals, alkali or alkali earth metals, and alumina. A generally accepted pathway for reactions of LNT is that under lean conditions, NO is oxidized to NO2, which is followed by subsequent formation of a nitrate with alkali or alkali earth metal(s), e.g. Barium. Under stoichiometric or rich operation, the stored nitrate is thermodynamically unstable. The stored NOx is released. The NOx then catalytically reacts with reducing species in the exhaust gas to form N2.
The alkali or alkali earth metal(s) that are typically used for NOx sorption are readily poisoned by sulfur oxides in the exhaust gas. Over time, the sulfur oxides react with these trapping elements forming sulfates which are more stable than corresponding nitrates. The present invention provides a new series of composite metal oxides that, combined with the active components of a LNT, significantly improve the sulfur tolerance of the LNT while maintaining or improving the other performances of the LNT.
The present invention solves this problem by providing a composition and method for storing and reducing NOx from lean burn internal combustion engines. The present invention uses composite metal oxides, in spinel structure, in conjunction with a typical LNT formulation to form an integrated LNT. The composite metal oxides in spinel structure act primarily as a SOx trapping element and also a secondary NOx trapping element within the integrated LNT. In the application of this integrated LNT, the sulfur oxides are mainly attached to the composite metal oxides in spinel structure under a relatively low temperature (200 to 600xc2x0 C.). When the integrated LNT is saturated with sulfur for a specified NOx conversion, the LNT will be desulfated under rich condition at a higher temperature (600 to 750xc2x0 C.), so that the capacity of the LNT for NOx trapping and conversion is regenerated. In this integrated LNT, the sulfur is trapped and released in a way that does not allow the sulfur to go to the primary NOx trapping element, i.e. the alkali or alkali earth metal oxides, to poison the integrated LNT; thereby, leaving more reactive sites for the NOx trapping and conversion.
The present invention provides a composition and method comprising a typical LNT formulation treated with a composite metal oxide in spinel structure. In one embodiment, the composite metal oxide spinel is in powder form and made into a slurry then coated into an LNT after all the other materials have been coated. Then the LNT is dried and calcined. In another embodiment, the spinel oxide slurry is mixed with CeO2 powder in an amount of 4-15 (wt) % of CeO2 of the total mixture before coating onto the LNT. In yet another embodiment, the spinel oxide slurry is mixed with CeO2xe2x80x94ZrO2 powder, in an amount of 4-15 (wt) % of the total mixture before coating onto the LNT.
The method for removing NOx and SOx impurities from exhaust gases provides a composition comprising an integrated LNT treated with a composite metal oxide having the spinel structure (AB2O4), and passing exhaust gas containing NOx and SOx over the composition. The NOx is stored under lean condition between 200 to 600xc2x0 C. and released and converted under stoichiometric and rich condition between 200 to 600xc2x0 C. Meanwhile, SOx is stored between 200 to 600xc2x0 C. under both lean and rich conditions and is desulfated at a temperature between 600 to 750xc2x0 C. under rich condition, when the trapped sulfur oxides have saturated the composite metal oxides and have thus reduced the NOx trapping efficiency to a predetermined value.